GB2045565A

GB2045565A - Protecting amplifier load against accidental short-circuits

Info

Publication number: GB2045565A
Application number: GB8008990A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1979-03-20
Filing date: 1980-03-17
Publication date: 1980-10-29
Also published as: FR2452203A1; IT1128045B; US4338573A; JPS6342882B2; FR2452203B1; NL7902160A; IT8067405A0; DE3010267C2; AU528102B2; JPS55127711A; GB2045565B; DE3010267A1; AU5654280A; ES489667A1; CA1135802A

Description

1 GB2045565A 1

SPECIFICATION

Amplifier circuit arrangement i 15 The invention relates to an amplifier circuit arrangement comprising first and second amplifiers having a common input and respective outputs for d.c.-connection to opposite ends of a load impedance, the first amplifier being inverting from the common input to its output and the second amplifier being non-inverting from the common input to its output. Amplifier circuit arrangements frequently employ non-linear negative feedback in order to protect the amplifier against overloading. As an example, United States Patent 4,006,428 describes a circuit arrangement which, in order to avoid distortion, employs non-linear negative feedback which controls the operating point of the amplifier in such manner that the amplifier always operates within its linear operating range. As another example Swedish Patent Application 7301979-6, which has been laid open for public inspection, discloses an amplifier arrangement in which non-linear negative feedback is used to prevent excess voltages from appearing on the amplifier, which voltages would damage the amplifier.

There is a problem when an amplifier ar- rangement of the kind set forth in the first paragraph is used, for example, as an output amplifier arrangement in a car, radio, in that if owing to carelessness or wiring defects one of the amplifier outputs comes into contact with the metal parts of the car (earth), a steady direct current of several amperes may flow through the load of the arrangement (a loudspeaker in this case). Loudspeakers are generally not capable of handling such a current and may be damaged thereby.

It is an object of the invention to provide a possible solution to this problem.

The invention provides an amplifier circuit arrangement comprising first and second amplifiers having a common input and respective outputs for d. c. connection to opposite ends of a load impedance, the first amplifier being inverting from the common input to its output and the second amplifier being non-inverting from the common input to its output, characterised in that a first d.c. path is provided from the output of the first amplifier to a d.c. bias point of the second amplifier and a second d.c. path is provided from the output of the second amplifier to a d.c. bias point of the first amplifier in such manner that the resulting d.c. paths from the output of the first amplifier to the output of the second amplifier and from the output of the second amplifier to the output of the first amplifier are each non-inverting, each of said first and second d.c. paths including a non-linear resistive element in series therewith, which element has relatively low and relatively high resistances when the low voltage across it is relatively high in a given sense and relatively low respectively.

An embodiment of the invention will be described, by way of example, with reference to the accompanying drawing, the single Figure of which shows the embodiment.

In the drawing an output amplifier circuit arrangement comprises first and second amplifiers 1 and 2, taking the form of high- power operational amplifiers, for example of the type TDA 1512 (Philips) or jointly accomodated in a semiconductor body as an integrated circuit. If they are of the type quoted the operational amplifiers 1 and 2 include output transistors 3, 4 and 5, 6 respectively, each pair of transistors being connected in series across power supply terminals (totempole arrangement). The amplifiers 1 and 2 have a common input which is connected to the output of an input signal source 7. Amplifier 1 is non-inverting from source 7 to its output 8 and amplifier 2 is inverting from source 7 to its output 9. Outputs 8 and 9 are therefore driven in anti-phase by the signal from the source 7, so that in turn the transistors 3 and 6 or 4 and 5 simultaneously become more conductive (bridge drive). A load, specifically a loudspeaker LS, is or may be included between the outputs 8 and 9 of the operational amplifiers 1 and 2 respectively, so that signal current flows through said load alternately in the one or in the other direction.

Unless steps were taken to prevent it, if one of the outputs 8 or 9 were to make contact with earth a large direct current would result through the load LS, with not only the risk that the loudspeaker coil would burn out, but also the possibility that the direct current would cause said coil to be deformed permanently. In order to prevent this, the outputs 8 and 9 of the amplifiers 1 and 2 are each connected to a d.c. bias point of the other amplifier, specifically the non-inverting input ( +) thereof, via resistors 11 and 12 respectively and a non-linear resistive element, comprising two semiconductor rectifiers 13 and 14 having a low internal threshold voltage connected in anti-parallel. By means of a voltage divider comprising resistors 15, 16 the common point of which is decoupled by a capacitor 18 these inputs are biased to a direct voltage which determines the standing d.c. voltage on the outputs 8 and 9. Generally this standing voltage is chosen to be half the supply voltage, because this allows transistors 3, 4 and 5, 6 to be driven the maximum amount. The direct connection shown from the inverting input of amplifier 1 to earth is in fact a connection for a.c. only, a d.c. bias voltage equal to half the supply voltage being supplied by means not shown to both this input and to the inverting input of amplifier 2.

It is arranged that during normal operation the direct voltage on the common point 17 of 2 GB2045565A 2 the resistors 15, 16 uis equazl to those on the outputs 8 and 9 so that the rectifiers 13 and 14 are cut off. This may be achieved in the - general case by comparing each output direct voltage (on points 8 and 9 respectively) with the input direct voltage (on point 17) in a comparator stage and deriving a negative feedback voltage from the difference, which feedback voltage is used to adjust the relevant output direct voltage so as to reduce said difference. (If the operational amplifiers are sufficiently wefl-matched with respect to their input off-set voltages, equality between their output and non- inverting input voltages may be obtained merely by suitably choosing the tapping point on the potentiometer 15, 1 6).If the resistors 11 and 12 are chosen to have equal values, the direct voltage at their common point will be free from a.c. signal compo- nents derived from the outputs of the amplifiers 1 and 2, in which case the decoupling capacitor 18 may be dispensed with.

If one of the outputs 8 (or 9) is now connected to earth, the rectifier 13 will become conductive and bring point 17 to a lower potential, so that via the d.c. coupling in the amplifier 2 (and 1 respectively) the other output D (or 8) is also brought to the same potential and the risk of overloading load LS is avoided.

Similarly, the risk that damage to the load LS may result from one of the outputs 8 or 9 coming intocontact with the positive terminal ( + VJ of thesupply voltage source, is avoided by the inclusion of the rectifier 14 in anti-parallel with the rectifier 13. All the circuit elements 11 to 16 may be incorporated in the same integrated circuit as the operational amplifiers 1 and 2.

Claims

1. An amplifier circuit arrangement comprising first and second amplifiers having a common input and respective outputs for d.c,.

connection to opposite ends of a load impedance, the first amplifier being inverting from the common input to its output and the second amp. lifier being non-inverting from the common input to its output, characterised in that a first d.c. path is provided from the output of the first amplifier to a d.c. bias point of the second amplifier and a second d.c. path is provided from the output of the second amplifier to a d.c. bias point of the first amplifier in such manner that the resulting d.c. paths from the output of the first amplifier to the output of the second amplifier and from the output of the second amplifier to the output of the first amplifier are each non- inverting, each of said first and second d.c. paths including a non- linear resistive element in series therewith, which element has relativeiy low and relatively high resistances when the voltage across it is relatively high in a given sense and relatively Jow respectively.

2. An arrangement as claimed in Claim 1, characterised in that the said resistive element comprises a semiconductor diode.

3. An arrangement as claimed in Claim 1, characterised in that the said resistive element also has a relatively low resistance when the voltage across it is relatively high in a sense opposite to said given sense.

4. An arrangement as claimed in Claim 3, characterised in that the said resistive element comprises a pair of semi- conductor diodes connected in anti-parallel.

5. An arrangement as claimed in any preceding Claim, characterised in that the resis- tive element included in each of the first and second d.c. paths is a single resistive element common to both said paths.

6. An arrangement as claimed in Claim 5, characterised in that the output of each ampli- fier is connected to the resistive element via a respective series linear resistor.

7. An arrangement as claimed in Claim 6, characterised in that the values of said linear resistorsare equal to each other. 90

8. An amplifier circuit arrangement sub- stantiaily as described herein with reference to the drawing.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd.-1 980. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained- h t